The tread of a modern tire is typically divided into a plurality of raised, load-supporting tread blocks separated by intersecting circumferential and transverse grooves and/or sipes. The tread blocks determine the control, acceleration and braking characteristics of the tire, while the grooves and/or sipes are necessary to provide flexibility and water removal.
The tread blocks are typically arranged in "base pitches", wherein each base pitch includes a predetermined geometry of whole and/or partial tread blocks. Each base pitch may vary in width across the tire. For example, each base pitch may extend transversely from one shoulder to the other shoulder of the tire, or it may extend transversely over only a single circumferential section of the tire, for example over a single rib or combination of ribs. In any case, the base pitches are repeated around the circumference of the tire to form the complete tire tread.
The constant contact of the tire tread on the road surface tends to produce unwanted noise. In particular, as the tire contacts the road surface, the individual tread blocks cause air disturbances upon impact with the road, creating a spectrum of audio frequencies broadly referred to as "tire noise". Tire noise is generated at least in part by: (1) the impact of the tread block on the road surface; (2) the vibration of the tire carcass; and (3) the "air pumping" which occurs as the tread blocks become compressed and expand into the grooves separating adjacent blocks. The fundamental frequency of the noise is a function of the number of tread blocks around the circumference of the tire and the rotational speed of the tire.
Techniques have been developed to distribute the noise frequency produced by the tire tread over a wide frequency band to approach what is termed "white noise". One known technique for reducing tread noise is to use base pitches having different pitch lengths, wherein the "pitch length" is a measure of the length from the leading edge of one base pitch to the leading edge of the next adjacent representative base pitch in the circumferential direction of the tire. A plurality of base pitches having different pitch lengths is conventionally referred to as a "pitch sequence".
For example, Lippman et al U.S. Pat. No. 2,878,852, discloses a tire tread having male and female mold halves, wherein each mold half has a separate pitch sequence extending around the circumference of the tire. Lippman discloses a pitch sequence which is represented by the series of relative circumferential distances: 9 10 11 12 10 11 12 13 12 11 10 9 11 12 13 10 10 13 12 11 9 10 11 12 13 12 11 10 12 11 10 9. These circumferential distances represent the relative circumferential length of successive tread units expressed in any desired measuring unit. In Lippman the tread units are selected to be mirror images of themselves or of other groups in each respective mold half.
Other techniques use random or sinusoidal sequencing of the pitches in an attempt to modulate the objectionable noise producing frequencies. For example, Vorih, U.S. Pat. No. 3,926,238, discloses a technique for modulating tread noise by providing discrete pitch lengths in which the ratio of pitch lengths falls within the open intervals defined by the boundary nodal points (N-1)/N:1.0, (N being an integer selected from 2, 3, 4, 5 or 6), and excluding the ratios defined by the boundary nodal points. Further modulation is obtained by sequencing the individual pitches in such a manner that the sequence is characterized by a plurality of strings of load carrying elements, each string consisting of at least three consecutive load carrying elements of substantially identical pitch length. The length and sequential positioning of the strings are selected to modulate the block frequency harmonic which is produced upon rotation of the tire.
Similarly, Landers, U.S. Pat. No. 4,474,223, discloses a method for reducing tire noise by spreading the noise generated by the tire tread over a broad frequency spectrum by:
i selecting the maximum number of repeating design cycles which may be placed about the apparatus; PA1 ii. selecting a maximum pitch ratio; PA1 iii. determining the appropriate number of harmonic segments in which the apparatus may be divided into in accordance with the following equation: EQU NS=[NP.times.(LP-SP)]/[B.times.(LP+SP)] PA1 where PA1 iv. determining the size of each of the harmonic segments and the number of design cycles for each of the harmonic segments; and PA1 v. arranging the different design cycle lengths in each of the harmonic segments so that the wave length of the predominant modulation frequency of the harmonic segment corresponds to the fundamental length of that segment.
NS is the required number of harmonic sequence PA2 NP is the number of pitches (design cycles) PA2 LP is the longest pitch length PA2 SP is the shortest pitch length PA2 B is the modulation index;
Accordingly, the above references disclose certain pitch sequences which are designed to reduce noise produced by tire treads by varying the pitch length of the tread blocks around the circumference of the tire to spread the noise over a broad frequency band. However, there is a constant demand in the industry for general techniques for selectively arranging base pitches in pitch sequences around the circumference of the tire to reduce the noise of the tire tread contacting the road surface.